(a) Technical Field
The present disclosure relates to a hybrid filler for an electromagnetic shielding composite material and a method of manufacturing the hybrid filler. More particularly, the present disclosure relates to a hybrid filler for an electromagnetic shielding composite material and a method of manufacturing the hybrid filler, which provides improved electromagnetic shielding and absorbing capabilities and which effectively removes heat generated by electromagnetic absorption.
(b) Background Art
Generally, electrical/electronic parts operated by power supplied from batteries, parts including small substrates having assembled thereon small elements such as resistors, and parts including combinations of mechanical apparatuses with such parts are all referred to as vehicle electrical/electronic parts.
Recently, the use of such electrical/electronic parts in vehicles has increased, particularly with the rising demand for eco-friendly vehicles, such as electric vehicles.
As the use of electrical/electronic parts rises, the need for materials having an electromagnetic shielding function to maintain safety and lifespan of the parts, and to prevent malfunction of the parts increases.
It is known that an electromagnetic wave causes malfunction of various electrical/electronic parts and is emitted or conducted from the parts. This can obstruct the performance of other devices, and may also cause changes in the human body such as stress, stimulation of the nervous system, and cardiac disorders.
Generally, an electromagnetic wave refers to a sort of “electromagnetic energy” generated in an electrical and magnetic flow. Upon generation of vibration when electricity flows, an electric field and a magnetic field which have different features are generated at the same time. The electromagnetic wave is a wave generated as the electric field and the magnetic field are periodically changed. Electromagnetic waves are inevitably generated in all devices using electricity.
An electric field (V/m) is generated proportional to the strength of a voltage and is easily shielded by a high-conductivity object. A magnetic field (mG) is generated proportional to the magnitude of a current and cannot be shielded due to its feature of returning to a region where it is generated.
Generally, an electromagnetic interference (EMI) shielding effect (EMI S.E.) is achieved when an electromagnetic wave incident to a test piece is reflected (R) from the surface of the test piece or absorbed (A) into the test piece, so that current is generated and transformed into heat through a resistor to pass through the resistor.
In most electrical/electronic parts, metallic materials are used to shield electromagnetic waves. When electromagnetic waves contact the metal surface, an eddy current generated by electromagnetic induction in a conductor reflects (R) the electromagnetic wave.
However, inter-path interference of an internal circuit board and interference with peripheral electronic devices can occur due to the reflected electromagnetic wave. As such, a material capable of shielding or absorbing the electromagnetic wave is required.
Use of a plastic part as an electromagnetic shielding material is advantageous in terms of moldability (processibility) and economical efficiency. Thus, the demand for such plastic parts has continuously increased due to its lightness in weight and high degree of freedom of design.
However, general plastic and plastic composite materials (structural composite) lack the conductivity of metal, and thus its adoption as a housing (case) of electrical/electronic parts for electromagnetic shielding is limited.
Because a plastic material has low crystallinity of a high polymer structure based on common coupling, an electromagnetic wave will completely penetrate the plastic material rather than become absorbed therein or reflected therefrom. Thus, research is actively being conducted to manufacture a composite material by dispersing within a plastic matrix a filler having superior conductivity to improve the EMI S.E. of the plastic material.
A conventional plastic composite material for electromagnetic shielding is manufactured by dispersing a metallic filler having excellent electric conductivity, such as silver, copper, etc., or a carbon-based filler such as graphite, carbon fiber, carbon nanotube, etc., with a volume content of 30% or more in a resin such as polyurethane, polycarbonate, epoxy, etc., so that a shielding effect of about 50 dB can be obtained.
A conductive filler used to manufacture the electromagnetic shielding composite material is mostly a metallic or carbon-based single filler. To shield an electrical field, a conductive filler of a high permittivity is favorable. To shield a magnetic field, a filler of a high permeability is useful. Therefore, for efficient electromagnetic shielding, it is desirable to manufacture a composite material by hybridizing fillers having such features.
Electromagnetic shielding for a composite material is described so that an electromagnetic wave incident as shown in FIG. 1 meets a conductive filler (steel fiber in FIG. 1) dispersed in plastic, causing multi-reflection or absorption, and thus the strength of the electromagnetic wave is attenuated and only a portion of the electromagnetic wave penetrates.
The absorbed electromagnetic wave causes movement of electrons of the filler dispersed in the matrix, generating current and resistance which are emitted as heat energy.
If the emitted heat energy is accumulated in the matrix or the emission speed thereof is slow, the shielding effect is degraded. Therefore, the EMI S.E. can be improved when the emitted heat energy is removed.
When a magnetic material is dispersed in the matrix of the composite material, a magnetic field absorption phenomenon occurs. This is due to a magnetic loss, in which an induced current is generated in the magnetic material due to the incident magnetic field and is transformed into heat energy (related document regarding EMI absorption of a magnet: IEEE Transaction on Magnetics, 44 (11), 2008, 3934-3937; J of the Korean Physical Society, 42 (6), 2003, 799˜802; J of Magnetism and Magnetic Materials 324, 2012, 1225-1229).
At present, work related to electromagnetic shielding is directed towards techniques which use a material having high conductivity alone.
U.S. Patent Application Publication No. 2006/0099403 mentions a technique for simultaneously using an electromagnetic shielding material and a heat radiant material, and suggests a method for manufacturing a composite material by mixing a shielding material, such as aluminum, iron, silver, magnetic, etc., with a heat conductive heat-radiant material, such as metal or ceramic powder, in a single layer.
However, in U.S. Patent Application Publication No. 2006/0099403, if 2 types of conductive materials (shielding material and heat radiant material) composed of spherical particles are not uniformly dispersed and hardened in an agglomerate form, both shielding and heat radiant features cannot be obtained at the same time. Rather, an electromagnetic shielding portion and a heat radiant portion are locally formed, respectively.